Understanding Bike Geometry Measurements

A guide to the terminology used throughout GeometryGeeks

Top Tube
aka: Effective Top Tube

Top tube is a good indicator of overall size of the bike.

It's measured 'effectively' horizontally from head tube axis to seat tube axis.

Older bikes frequently had horizontal top tubes, but now sloping tubes are much more common. Where the top tube is horizontal the Effective Top Tube and Actual Top Tube will be identical.

A longer top tube has you more stretched out on the bike, all other things being equal. This will give you a racier, more aerodynamic position on a road bike - possibly at the expense of all-day comfort.

In mountain biking, top tubes have been lengthening as stems get shorter and bars wider, giving a more stable ride at speed.

Actual Top Tube Length

Top tube length as measured from head tube axis to seat tube axis, along the tube itself. This is not particularly useful for bike fit.

Seat Tube Length

Length from bottom bracket centre to top of seat tube.

Head Angle
aka: Steerer Angle

The angle the forks make with the ground.

A slacker (smaller) head angle gives more stable handling at speed, while steeper (larger) makes a bike more precise at slow speeds.

Seat Angle
aka: Seat Tube Angle, Effective Seat Tube Angle

Seat tube angle. On mountain bikes where seat tube does not intersect the BB, the 'effective' angle.

Actual Seat Tube Angle

On mountain bikes where seat tube does not intersect the BB, the actual angle.

Reach

The horizontal distance from the centre of the bottom bracket to the centre-top of the head tube, where the virtual steering axis passes through.

Stack

The vertical distance from the centre of the bottom bracket to the centre-top of the head tube, where the virtual steering axis passes through.

Head Tube

Length of head tube.

A tall head tube may mean you can't fit forks that have had their steerer tube cut, such as those from a smaller bike. Make sure the fork steerer is long enough to allow for headset cups, stem height and head tube length.

Chainstay

Distance from rear wheel axle to bottom bracket centre.

Wheelbase

Distance between front and rear axles.

Standover
aka: Standover Clearance, Standover Height

Standover height or clearance is the height from the ground to the top tube. Normally measured at a point halfway along the top tube.

Front Centre

Distance from centre of bottom bracket to the front axle.

BB Drop

How far the bottom bracket is below a line drawn between front and rear axles.

Fork Rake/Offset

How far in front of the steerer tube axis the front axle is.

Trail

The distance between the point where the projected steerer tube axis meets the ground and the point where the the wheel contacts the ground.

Seatpost Size (Diameter)

Diameter of seatpost required for the frame. Some frames may require specific shapes of seatpost (e.g. aerodynamic profile) or use a fixed seatmast.

Seatpost Length

Length originally specified as part of the complete bike, or a dropper post.

Handlebar Width

Width originally specified as part of the complete bike.

Stem Length

Length originally specified as part of the complete bike.

Crank Length

Length originally specified as part of the complete bike.

BB Type
aka: Bottom Bracket Type

Bottom bracket standard, e.g. Pressfit BB30 or Threaded 73mm shell.

Axle Spacing

Rear axle spacing standard, e.g. 12x148 Boost or 135x9 QR.

BB Height

Height of bottom bracket centre above ground. Varies depending on tyre choice, suspension sag and other factors - this is as quoted by the manufacturer.

Axle to Crown
aka: Fork Length

The distance from front axle to top of fork crown. May also be referred to as fork length, normally on road bikes (but not technically the same thing).

Wheel Size

Diameter of rim used, and tyre width if appropriate. e.g:

  • 700c road wheel
  • 26"/27.5" (650b)/29" MTB wheels
  • 29+ semi fat.

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